Pressure vacuum breaker (PVB) valves are used to protect against back-siphonage in a wide variety of conditions such as in industrial plants, laboratories, laundries, swimming pools and lawn sprinkler systems. PVB valves prevent back-siphonage of water into a potable water supply. The PVB valve includes a one-way check valve which closes tightly when flow through the valve drops to zero, and an air relief valve that opens to break the siphon when pressure drops to 1 psi, thereby preventing back-siphonage. The PVB valve may also include a spring-loaded relief valve that, in the event of exposure to freezing temperatures, opens to release freezing water and protect the PVB body and internal components from damage. As the temperature increases above freezing, the relief valve automatically closes.